Electric temperature control



Dec. 31, 1963 K. J. KNUDSEN ELECTRIC TEMPERATURE CONTROL 2 Sheets-Sheet 1 Filed Feb. 1. 1961 n R m 6 i K i? \g 3 J or ma m K i 2 Q09 5. 9 9 7 2 \W @m R K- J- KNUDSEN ELECTRIC TEMPERATURE CONTROL Dec. 31, 1963 2 Sheets-Sheet 2 Filed Feb. 1, 1961 United States Patent Ofiice Fatented Dec. 31, 1963 3,116,396 ELECTRTC TEMPERATURE CGNTRGL Knud .ll. Knudsen, Middlebury, Comm, assignor to Lewis Engineering @ompany, Naugatuck, Conn, a corporation of Connecticut Filed Feb. 1, 1961, Ser. No. 86,432 6 Claims. (Cl. 21920) This invention relates to electric temperature control devices which are automatically operative, and more particularly to devices of this kind wherein the energization of an electrical heating means such as a resistance wire, heating element or the like is automatically controlled in accordance with the temperatures produced by such heating means.

The invention involves improvements in the automatic electric temperature control devices described and claimed in my Patent No. 2,761,052 dated August 28, 1956, and entitled Electric Temperature Control. In this patent there are described temperature control devices wherein the circuit of an electric heating element is controlled by means including a vacuum tube to automatically effect energization and de-energization of the element for the purpose of maintaining the temperature of a medium such as an oil bath or the like, at a constant value. The temperature of the oil bath is sensed by a sensing element such as a thermocouple or resistance thermometer bulb, which is connected in a bridge circuit having in one form of the invention an electric instrument movement such as a galvanometer, which then is responsive to changes in the temperature of the oil bath.

in this form of the invention of my patent identified above the galvanometer is provided with a shutter by which light from an electric bulb may be controlled to energize or de-energize a photoelectric cell. The conductivity of the cell is thus a function of the movement of the galvanometer and accordingly reflects the changes in temperature of the oilbath. The photoelectric cell is connected in the grid circuit of a pentode vacuum tube preamplifier which is thus responsive to the conducting and non-conducting conditions of the cell. The pentode tube in turn is connected so as to control a Thyratron vacuum tube having in its plate circuit a power transformer of the auto-transformer type, which in turn is included in the circuit of the heating element to control the energization thereof.

In another form of the invention described in my patent neither the photocell nor the galvanometer is utilized but instead the unbalance of the bridge operates a transformer which controls the first grid of a double-triode preamplifier vacuum tube, by which an amplification of the control signal is obtained, the said double triode vacuum tube in turn being connected so as to control the Thyratron in the circuit of the heating element, thereby ultimately to control the energization of the latter.

In these patented systems it was necessary to utilize in conjunction with the Thyratron tube an additional vacuum tube control and amplifier means in order to efiect a reliable control over the temperature of the oil bath when utilizing the relatively weak signal which is normally manifested at the bridge circuit connected with the thermocouple or resistance bulb. In one case a pentode vacuum tube preamplifier was utilized whereas in the circuit not having the galvanometer a double triode vacuum tube preamplifier was employed.

These arrangements involved additional components in the form of the added vacuum tubes and supplementary circuit elements, transformers, power supplies and the like, making the equipment more bulky and costly.

The above disadvantage of the devices of my identified patent are obviated by the present invention, and one object of the invention is to provide a novel and improved automatic temperature control device of the general type eliminating make-and-break contacts by use of a Thyratron vacuum tube, wherein no further vacuum tube devices in addition to the heater-controlling Thyratron are required. Thus, there is effected an appreciable simplification of the control device, a reduction in the number of components and a reduction in cost, together with improved reliability since fewer components exist which might be subject to breakage, failure and the like.

In accomplishing this the invention provides a novel rectifier-type energized and polarized signal circuit having a charged or polarity signal supplying capacitor for connection with the control grid of the Thyratron tube, such signal circuit utilizing a high current capacity, switching-type photosensitive cell capable of fast switching action involving appreciable values of current. The signal circuit embodying the said photosensitive cell is arranged to provide not only signals of opposite polarity selectively on the control grid of the T hyratron in a reliable manner without requiring the use of additional vacuum tubes and the associated components required therefor, but involves further a novel, capacitor-potential action to effectively render the Thyratron grid less negative so that conduction of the tube is quickly established.

A further object of the invention is to provide a novel and improved temperature control device as above outlined, which may utilize either a thermocouple type sensing element or else a resistance bulb type element, in sensing the temperature which is to be kept constant or controlled.

Another object of the invention is to provide, in an electric temperature control device employing a vacuum tube valve, an improved DC. control circuit energized from an alternating current source and characterized by a charged filter capacitor adapted for connection through a conductive-type photosensitive cell to a control grid, which circuit in conjunction with AC. energization of the vacuum tube provides a reliable and sensitive, fastacting response of the tube to input signals of even small magnitude.

Still another object of the invention is to provide an improved combination comprising an electric temperature control involving a vacuum tube valve device and a time-delay relay, by which the filament or heater of the vacuum tube is given time to obtain its operating temperature prior to the application of signals to the tube.

Yet another object of the invention is to provide, in an electric temperature control, an improved Wheatstone bridge network having constants in the form of resistive components which are so related to each other that the same amount of error occurs at both the low and high limits of the operating range, independently of lead resistance of the temperature responsive or sensing element, within limits.

A feature of the invention resides in the provision of an improved control device as above set forth, which involves relatively simple, few components and which is extremely reliable in its operation at all times.

Other features and advantages will hereinafter appear.

In the accompanying drawings:

FIG. 1 is a schematic circuit diagram of one form of improved temperature control device as provided by the invention.

FIG. 2 is a schematic circuit diagram showing another form of the present improved temperature control device.

FIG. 3 is a schematic circuit diagram showing a simplification of the bridge circuit of FIG. 1 and indicating one extreme setting of the indicating slider corresponding to the low temperature value of the sensing element, which may for example be 50 centigrade.

IG. 4 is a schematic circuit diagram similar to that of FIG. 3 but showing the indicating slider at the opposite end of its path of travel, corresponding to the highest temperature reading, which may be for example 310 Centigrade.

Considering first the circuit shown in FIG. 1, power supply terminals are indicated at iii and 12. The terminal 12 is connected to a suitable common return or ground bus, indicated as a ground 14. The terminal it? is connected through an on-off switch 16 to a lead 18 which is connected with an electrical heater device 2i), the latter in turn being connected with a winding 22. of an autotransiormer 24, said winding being also joined with a ground return 2". The autotransformer 24 has a control winding 28 connected through an ammeter 3i! to the Winding 22 as shown.

With this arrangement, if a switching connection is established between the remaining terminal 32 of the winding 28 and the ground by a switch device connected with ground, such as a Thyratron tube indicated at 34 in the diagram, a control will be established over the energization of the heater 2% from the suppiy terminals 16, 12.

An indication as to the on-oti condition of this system and of the switch 16 is given by an indicator lamp 35 having one terminal connected to a ground. 36 and having its other terminal connected through a resistor 3%; to the hot lead 18.

The Thyratron 34 is shown as havins a plate or anode it? connected with the lead 32, and as having a screen grid 42 connected by a wire 44 with its cathode or reference element 46, the lead 4 being also connected to a ground return 43. The heater of the Thyratron tube is energized from the secondary 52 of a filament transformer 54 having a primary 56 connected to a ground 6% and to a hot ead 62 joined to the lead wire 15. The Thyratron tube 34 has a control element or grid 64, on which the control signal is impressed.

The heat from the electric heater 2% is sensed by a resistance bulb 65 connected, by means of a cable comprising wires 75), 72 and '74 to a bridge circuit which is designated generally by the numeral 76. The wire 74 is connected to a ground '78 as indicated. The resistance bulb 55 comprises one leg of the bridge 7% being connected by the wire '72 with a resistor 8%), comprising a second leg. The two remaining bridge legs comprise resistors 32 and 84, the latter being connected to the lead wire 7% through a vernier resistor 86 which may have a low value, as for example 2 ohms. The resistors 56*, 82 are connected with the resistance element 58 of a potentiometer device having a slider or contact arm 3% which is connected to the galvanometer or instrument movement 92. The movement 92 is connected to an adjustable contact arm M which is movable over the Vernier resistor The resistance legs 82 and 84 are connected respectively to an anticipator device 96 the functioning of which will be described in detail at a later point, comprising resistors 98 and 1% which are engageable simultaneously by a slider N2 whereby an adjustable connection is eil'ected between the resistors and litiib.

In accordance with the present invention, in conjunction with the AC. energized Thyratron tube there is provided an improved energized, rectifier-type DC. control or signal circuit having a high-current, conductive photosensitive cell, which ettects a rapid and reliable response of the Thyratron at all times, even to signals of relatively small magnitude, the said control circuit being charact e rTz ed by a charged, high-capacity filter condenser which can be directly connected to the Thyratron control grid through the photosensitive cell to drain the grid of its negative charge or electrons by connection with its positive terminal, thereby to insure quick conduction of the tube for the intervals of positive polarity of the anode thereof whenever a signal is present in the control circuit.

The said control circuit may be triggered by a strong light beam which, as illustrated in FIG. 1, is under the control of movement of the galvanometer or instrument movement 92.

As shown, the polarized control circuit providing the grid of the Thyratron tube 34- with the desired signal, comprises a photosensitive cell 168 of the type which becomes conducting in the presence of light and is capable of carrying appreciable current. The cell 103 is herein termed a conductive-type photosensitive cell, and may be that produced by the JEM Manufacturing Co., model or type COS10. The cell 1% is used as a switching device in conjunction with a charged capacitor connected in the signal or grid input circuit. Such circuit also comprises a grid resistor, together with rectifier means for the purpose of effecting either a negative or a positive polarity on the control grid 64 of the tube As shown, the DC. power supply for the signal circuit comprises a transformer lit-P having a primary winding n2 connected to a ground H3 and also to a hot supply line 131 which latter is joined to the supply wire 18 and has in series with it a voltage regulator device 116 such as that known commercially as an Amperite voltage regulator. The transformer 116% has a seconding winding connected with a lead 12%, also called herein a reference voltage point, going to the wire and ground The winding T18 is also connected with wires 124 and 1% leading to rectifiers 1.2-8 and 13% respectively, the rectifier-s being connected to positive and negative-potential or voltage wires 132 and 134 respectively. From the wires 132 and 134, high-capacity filter condensers T36 and 135i lead to the grounded wire 12%. The positive suppiy wire 132 is connected to the photosensitive cell which is in turn connected by a wire 135 to the control grid 64 of the Thyratron tube 34. The grid lead 135 is also connected to a grid resistor or voltage drop device 139 which is joined to the juncture of the resistors 84 and 98. Also, the resistor 9-8 is connected to the negative wire 1343 of the signal circuit.

With the above organization, when the photosensitive cell res is not subjected to light it will be essentially nonoonduoting, whereupon the negative potential on the wire 7.34 will prevail and be applied to the control grid 64 of the Thyra ron: tube 34 through the resistors 98 and 139, making the grid 64 negative with respect to the ground 48, and thus with respect to the cathode 46. The plus terminal 137 of the capacitor 136 will have an appreciable positive polarity, the capacitor being charged by the coil 118 and rectifier 128. Accordingly, the Thyratron will not conduct, for the above conditions, during those intervals when it is capable of conduction due to the anode 46) thereof being positively polarized by the alternating current applied to the power supply terminals 1t 12. if, however, the photosensitive cell 108 is subjected to an appreciable illumination it will be rendered conducting whereupon the positive potential on the wire 132 and terminal 137 of the capacitor will be applied to the Wire 1135 and will prevail over the negative potential manifested through the resistors 93, 139. That is, the resistors 93, 139 will now constitute an active load on the DC. supply and capacitors 135 and 138. Accordingly a positive potential will be applied to the control grid 64- instead of the negative potential above mentioned. The resistors 98 and 13% are so arranged that for such circumstance they are able to withstand the application. of the potential represented by the Wire 135 being positive and the wire 13d being negative.

For such positive potential on the grid 64 the Thyratron 34 is rendered conducting, whereupon it will eifect a closing of the circuit through the transfonmer coils 22, 28 in a manner such that the coil 28 causes a loading of the transformer, which reduces the voltage drop across the coil 22 and effects an energization of the heater 20 from the terminals 1t 12. It will be understood that the above negative potential on the control grid 64 when the light cell W8 is not illuminated maintains the Thyrratron tube 3 5- in a non-conducting condition, whereupon the windings 22, 28 of the autoatnansformer 24 remain in effect open-circul ted, thereby enabling the voltage of the source l9, 12 to be absorbed mostly by the transformer winding 22, resulting in little if any energization of the electric heater 2d.

As further seen in FIG. 1, these is provided a light Control means comprising a slotted plate Mil having an aperture 1 22, said plate being carried by a pivoted arm 144- which may, for example, comprise a portion of the movable component of the galvanometer or instrument movement 92. In H6. 1 a broken line 146 indicates a mechanical connection between the arm 142 and the galvanometer movement 92 whereby a response of the galvanometer to an unbalance of the bridge '76 will eilect a turning movement of the slotted plate 146). The plate Mil is arranged in front of the photosensitive cell 108, and incandescent bulbs M3 and 15% are placed in line with the slotted plate and the photo-cell 1% whereby light from the bulbs may pass through the aperture 1'4?r in the plate and strike the cell 198 when an unbalance of the bridge '76 due to low temperature of the element 65 deflects the galvanometer 92 in one direction. That is, the arrangement is such that for deflection of the galvanometer or instrument movement 92 due to a bridge unbalance resulting from the sensing element 65 being in a cold state, the slot 142 will be aligned with the bulbs 148, 159 and the photosensitive cell 163 whereby the latter will receive full illumination from the bulbs. This will render the Thyratron 34 conducting. And, when the medium being heated and the sensing element 65 are at the desired temperature the bridge circuit will balance and bring the galvanometer 92 to a position wherein a solid portion of the plate 140 obstructs light trying to reach the cell 168 firorn the bulbs 143, 150.

By the present invention, a novel and simplified time delay mechanism is associated with the bulbs 148, 150 in order to delay the energization of such bulbs at the time that the apparatus is first turned on. Thus, by delaying the lighting of bulbs 1-48, 150 the Thyratron is prevented from being subjected to a signal which could place it in a conducting condition prior to the heater 5i) thereof fully heating the cathode 46 to provide the necessary electron emission.

The time delay device comprises a bi-metallic switch arm 15% which is arranged in close pnoxirnity to a heater 158, the latter having one terminal connected by a wire ass to the ground M3. The other end or the beater 158 for the biamotallic arm 156 is connected by a wire 162 to a stationary contact 164 of a relay 166, such relay having a coil 16% connected to the ground wire 16b. The other end of the relay coil 16% is connected to a station ary holding contact no of the relay, such contact and the first-mentioned stationary contact 164 being cooperable with the relay armature 1'72, which latter is connected by a wire 174 to the hot wire 114 of the system. The bulbs 143, 15th are bridged by a resistor 176, and. one set of terminals of the bulbs and resistor is connected to one end of a ctnrent-limiting resistor 136 the other end or which is connected by a wire 178 to the bimetallic switch arm 3156. The remaining terminals of the bulbs and the resistor 176 is connected by a wire 180 to the ground wire lei). Cooperable with the bimetallic switch arm 1% is a stationary contact ill-ll which is connected by a wire 18 3 with the hot line 114. Finally, the stationary contact 17d of the relay 1.66 is connected to the wire 178. By this arrangement the closing of the switch '16 effects an energization of the heater 15% which, after a short period, causes a flexing of the bi-metallic arm 156 Whereby the relay coil tea is energized as well as the bulbs 148 and rise. The interval of delay gives the heater-type cathode d of the Thyiwatron time to become fully heated.

Operation of the temperature control circuit shown in FIG. 1 is as follows:

from the hot wire 114 and through the wires 184,

178, 186 and resistor 186 to the ground 113. At the same time the relay coil 168 will become energized, thereby attracting the armature 1'72 and effecting a tie-energization of the heater 158. Also, the relay armature 172 15 will now engage the stationary contact 170, thereby closing a holding circuit for the relay coil 160 through the wire 174. Accordingly, the biametallic alum 156 will now have an opportunity to return to its initial, open circuit position. Energizaltion of the lamps 148, .150 will be 20 continued through the current limiting resistor 186 and through the holding circuit comprising the relay armature 172 and the supply wire 174.

Assuming that the heater 20 is in a cold state, as well as the sensing element 65, the bridge 76 will have an unbalanced condition whereby the deflection of the galvanometer 92 will cause the slot 142 of the plate 140 to be aligned with the bulbs 148, 15d and the photosensitive cell 108. The cell res will therefore be fully lighted, and its conducting condition will render the Thyratron grid 64 highly positive whereby the Thyratron will be made conducting. In consequence, the autotransformer 24 will function as a closed switch, causing full energizetion of the heater 26 As the latter raises the temperature of the oil bath or other medium, the sensing element 65 will become hetaed, resulting in increasingly less deflection of the galvanometer as the temperature rises. This will result in less and less illuminataion of the photocell llld in consequence of the slot 142 of the plate 149 being moved out of alignment with the lamps 1 58, 150

and the photosensitive cell.

The result will be a decrease in the positive potential on the control grid 64 of the Thyratron tube 34, thereby reducing the plate current of the Thyratron which has the effect of increasing the impedance of the auto-transformer in the heating circuit of the heater 26. Accordingly, the

energizing current through the heater 20 will be reduced as the surrounding temperature becomes higher.

A point will be reached in the heating of the sensing element 65 Where the balance of the bridge 75 will cause the galvanometer 92 to completely shut off light from the bulbs 148, 150 to the photosensitive cell 108. When this has occurred the Thyratron 34 is no longer conducting, and in consequence the auto-transformer 24- functions as an open switch, terminating for all practical purposes the energization of the heater 20. Depending on where the cut-off point is for the Thyratron 34, such tube may be rendered non-conducting prior to complete shutting off of the light from the photosensitive cell 168, the controlling factor being the cut-off potential of the grid 64. With no light striking the photosensitive cell M8, the grid 64 will be essentially negative as a consequence of the potential suplied by the transformer coil 118 in conjunction with the rectifier 130 and filter capacitor 133.

With the photo-cell 1% fully illuminated and conducting, the grid 64 will have essentially a positive po tential as supplied by the transformer 11% in conjunction with the rectifier 128 and capacitor 136. At some point between the full positive and full negative potentials supplied to the grid. 64 there will be a cut-oil, Where the tube is rendered non-conducting from its conducting condition or is rendered conducting from its non-conducting condition.

Thus, by the above organization a lowering of the temperature of the heater 2t) and sensing element will tend to effect illumination or a greater illumination of the photosensitive cell 198 as controlled by the bridge 76 and galvanomcter 92 together with the shutter 14%), the increasing illumination bringing the control grid 64 more positive from its previous value whereby the Thyratron Se is made conducting or is rendered more conducting. This will then energize the heater 29 to again raise the temperature, and the above cycle will be repeated continually, resulting in a close control of the temperature at the heater 20.

The anticipator action of the device 96 is as follows: Depending on the setting of the contact arm an or" the anticipator device, a greater or less voltage will be introduced into the bridge circuit whenever the photosensitive cell 1698 is conducting. If the arm N2 is more to the left, when viewing FIG. 1, the voltage introduced will be greater whereas if the arm 162 is more to the right, the introduced voltage will be less. For any setting of the arm Hi2, however, the total resistance represented by the resistance elements 98, 1139 which is included in the bridge circuit will remain the same and will be a constant. The voltage introduced into the bridge circuit by the anticipator device 96 is in such a direction as to tend to decrease the unbalance of the bridge and to decrease the deflection of the galvanometer 92. With the arm m2 at the extreme right, the least amount of voltage is introduced, and thus there is the least opposing action to unbalance of the bridge and deflection of the galvanometer. However, when the arm N2 is located at the left end of its movement, the greatest amount of voltage is introduced into the bridge during the intervals of conduction of the cell 1%, such voltage opposing to the greatest extent the unbalance and reducing the same, as well as the deflection of the galvanometer 92. This action has the effect of preventing overshooting of the galvanometer and control, and of preventing hunting. A detailed explanation of this type of anticipator control is found in my patent above referred to, No. 2,761,052 dated August 28, 1956.

Another embodiment of the invention is illustrated in PEG. 2, wherein the sensing device comprises a thermocouple as distinguished from the resistance bulb sensing element of FIG. 1. In FIG. 2 a diiierent kind of time delay device is shown, and a somewhat different polarized signal or grid input circuit involving a conductive-type photosensitive cell of the kind shown in FIG. 1.

Considering FIG. 2 specifically, there are illustrated power supply terminals 2% and 2632, the latter being connected to a ground 294. The terminals 2%, 262 are also connected to a double-pole single-throw, on-ofi switch 2%, to which there is further connected a second ground 203 as shown. The switch 2% has a hot line 2M) connected to a fuse 212 which is in turn connected to a hot line 214 joined with a relay contact arm 216.

Cooperable with the arm 216 is a fixed contact 218 connected with a load or heater 22% which is joined to one set of terminals of parallel-connected coils 222 and 224 of an auto-transformer 226, the remaining terminals being connected to a wire 22% joined to a ground 230, constituting the return of the power circuit for the heater 220.

The auto-transformer 226 has a control winding 238 connected through an arnrneter 240 to the load windings 222, 224. The control winding 222 is also connected by a wire 2 .2 to the anode 2 24 of a Thyratron tube 24-6, by which operating polarity is brought to the anode as well as a connection being made with which the tube may effect a switching action to control the loading of the autotransformer 226.

The Thyratron 246 has a screen grid 248 connected by a wire 2% with a cathode 252, and has a heater 254 connected with the cathode 252 and energized from the secondary 256 of a filament transformer 260, such secondary being also connected to an incandescent bulb or light source 2152 provided for use with the conductive photosensitive cell of the control. The transformer 260 has a primary coil 264 connected to a ground 266 and also to a hot Wire 268 which joins the wire 214 mentioned above. Thus, the heater 254 of the Thyratron becomes energized, as well as the light source or lamp 2262 upon the switch 2% being closed. However, closing of the switch 2% will not result in an operating potential being applied to the anode 24-4 of the Thyratron inasmuch as the switch 216 controls this part of the circuit. Therefore, the Thryratron will not be polarized for operation, insofar as its anode is concerned, at the time that the heater 254 is connected and is beginning to warm the cathode.

The thermocouple bridge is designated generally by the numeral 288, such bridge having a pair of resistor legs 290, 292 provided with a common connection leading to a switch 2% across which an ammeter 293 is connected. The switch 296 and ammeter 298 are joined by means of a wire 3% to the movable arm of a rheostat 3%, which is in turn connected to the power supply wire 3&6 of the bridge, said wire constituting part of a transistor-type voltage control circuit 3%. Such voltage control circuit is described and illustrated in my copending application Serial No. 656,422, filed May 1, 1957 and entitled DC. Regulator and Con trol Circuit (now Patent No. 3,004,209 dated October 10, 1961). It will be described here only briefly, in consequence.

The voltage regulator 263 comprises a transistor 31% having a base 312 connected to the junction of a crystal diode 31 i and resistor 316, the latter being connected by a wire 313 to the emitter 32d of the transistor. Th collector 32-2 of the transistor is connected through a resistor 32% to the output wire 3%.

The crystal diode 3?.4- is connected to the juncture of two resistors 32% and the latter being bridged by a rheostat 33th and being also connected to the output wire 3%. The resistor 326 is connected to an input wire 332, also termed a positive voltage point, which receives a positive potential by virtue of connection to rectifier-s 334 and 336 having a common junction 333. The rectifiers are connected to the ends of a center-tapped secondary winding of a transformer having a primary winding connected to a ground lead and to a hot line 34? joined to the wire 263.

The center tap 359 of the secondary 349 constitutes the negative voltage point or terminal of the DC. supply circuit, and is connected to a lter capacitor which is in turn connected to the juncture 333. Also, the center tap 356 is connected by a wire to a pair of series-connected resistors 35%, 358 having a common juncture, also termed herein a reference voltage point, connected by a wire to the ground The resistor 358 is connected to the juncture 333 as shown. The center tap 359 is also connected through a wire 361, anticipator potentiometer device 362, wire and a dropping resistor 364 to the control grid 37% of the thyratron tube 246. The potentiometer am has a resistance element 365 of about 2 ohms, and a contact arm 366, which latter is joined to the wire The grid signal wire 363 is connected to the photosensitive cell 372 by a wire 374-, by which a positive potential is established on the grid 372 when the cell is rendered conducting. The cell is in turn connected with the positive supply line 332 polarized by the rectifiers 33d, ass and charged by the capacitor 352.

With the above organization, the absence of light on the photosensitive cell 372 will result in its being nonconducting, whereby the negative potential at the center tap 55%) of the transformer secondary 3% will be applied to the control grid 370 of the thyratron 346, preventing conductive operation of the tube. No current, of course, will be flowing in the wire 363 for such condition. If, however, light strikes the photosensitive cell 372 so as to render it conducting, a positive potential from the line 332 and capacitor 352 will be applied to the control grid 3'70, overcoming the ne ative polarization through the resistor 364. This will cause the thyratron to conduct, whereupon it will load the auto-transformer 226 and cause the latter to function as a closed switch. The dropping resistor 364. is arranged to withstand the difference of potential between the negative line 354 and the positive line 352, when the cell 372 is conducting, as will be understood.

The sensing element which is arranged to respond to the temperature of the heater 22% comprises a thermocouple 378 which is located either in proximity to the heater 220 or else in heat-conducting relation thereto, in any desired manner. The thermocouple 378 has terminal wires 33% and 382, the latter being connected to a cold junction compensator comprising a circuit set apart by the broken rectangular outline 384. This compensator may comprise one of the legs of the bridge, and includes parallel-connected compensating resistors 386 and 333 consisting of copper and nickel, together with a cold junction comprising Alumel and copper wires 39-9, 392. The bridge 2% has a fourth resistive leg 3% connected to the element 365 of the anticipator potentiometer by a wire 367. The element 1%5 is connected to a common junction 369 of the parallel resistors 386 and 388.

The bridge further has a six-ohm slide wire potentiometer device sea, the ends of the slide wire being connected respectively to the legs 2% and 3%, said device having a slider 4tl2 connected to a galvanometer ens which is in turn connected to the thermocouple terminal wire 386 by a wire 495. The thermocouple loads 336, 362 may be respectively of Chromel and Alumel, and the wire 4% may be of copper.

The control further has a shutter comprising a plate 4% having an aperture did, the said plate being carried by a pivotal arm 412 which is shown by means of the broken line 414 as having a mechanical connection with the galvanometer or instrument movement 4%. The arrangement of the shutter, galvanometer and photosensitive li ht cell is generally the same as that described above in connection with FIG. 1. When the heater 2i) and thermocouple 378 are in a cold condition, the galvanometer 404 is deflected by virtue of the bridge being unbalanced, and such deflection brings the aperture 41%) of the shutter 408 in line with the photosensitive cell 372 and the light bulb 262. Accordingly, the photosensitive cell will be illuminated, and its conductive condition will render the thyratron 346 conductive. As the temperature of the heater 229 and the thermocouple 373 rises, the deflection of the galvanorneter 464 will become less whereby light will be gradually shut oil from the photosensitive cell 372, thereby rendering the latter less and less conducting to the end that the positive potential on the control grid 370 of the thyratron will be reduced or changed toward negative, ultimately shutting off the thyratron. Thus, the control action is similar to that already described above.

By the invention a novel combination comprising the above control means in conjunction with a time delay is provided, to put oft the supply of positive operating potential to the anode 2444 of the thyratron 2% until the heater of the tube has become thoroughly warmed and is emitting electrons. The delay device comprises a motor 418 having one terminal connected by a wire 42% to the ground W8, and having its other terminal connected by a wire 422 to a contact 424 which is cooperable with the relay arm 216 (alternately with the contact 218). The relay, indicated generally by the numeral 5%, has a second set of cooperable contacts 428, 4-30 joined respectively by wires 432 and 434 to the ground wire 420 and to a switch contact 4-36 associated with the motor 418. A movable switch contact 433 is connected to the wire 432 and is arranged to be engaged with the contact 436 after the motor 41% has completed a certain number of revolutions, upon its being energized. The relay 426 has a coil 442 connected at one end with the hot wire 214 and at the other end with the wire Operation of the control device shown in FIG. 2 is as follows: The on-oil switch 206 is first closed. This results in energization of the transformers 342 and 366. Energization of the latter transformer eifects energization of the Thyratron heater 2% as well as the light bulb 262. Energization of the transformer 342 provides the DC. potential between the wires 332 and 354, in the signal control circuit of the Thyratron tube 246. Also, closing of the switch 2% eifects energization of the motor 418, and the latter begins its rotation. After a series of revolutions the motor 418 will close the switch comprising the contacts 436, 433. This delay interval is sufficient to enable full heating of the Thyratron heater and cathode 254, 252.

Closing of the switch 436, 4-38 now energizes the relay coil 442, whereupon the relay contacts are shifted, thereby connecting the heater 22th in its energizing circuit which includes the auto-transformer coils 222, 224, and the ground 23%).

Since at first the heater 220 is cold, as well as the thermocouple 373, the galvanometer 4% will be deflected and the aperture 410 of the shutter 4'98 will be aligned with the light bulb 262 and the photosensitive cell 372. Accordingly, such cell will be rendered conducting, and the positive polarization of the terminal 338 of the capacitor 352 will be brought to the control grid 376 to pull off the charge and render the Thyratron 246 conducting, whereupon this latter will load the auto-transformer 226 to cause the coils 226, 224 thereof to exhibit very low impedance so that the transformer will function as a closed switch. Accordingly, the heater 220 will be energized, raising the temperature of the oil bath or the like and also the temperature of the thermocouple 378.

As the temperature of the latter rises, the deflection of the galvanometer 464 will be reduced, thereby reducing the illumination of the photosensitive cell 372 and reducing the positive potential impressed on the con trol grid 370. The action of the Thyratron will thus be similar to that already described in connection with FIG. 1.

When the photosensitive cell 372 is no longer conducting, the grid 370 will be negative with respect to the ground and the cathode 252, essentially by the amount of voltage drop existing across the resistor 356, and for such condition the Thy-ratron tube 246 will be nonconducting, whereupon the energization of the heater 220 will be discontinued.

The inclusion in the bridge 288 of the potentiometer device 362 constitutes an adjustable anticipator device which has a number of advantages. When the Thyratron 246 is rendered conducting by illumination of the cell 372 the circuit established through the latter includes the potentiometer 362. For, from the positive point 332 current will flow through the cell 372, wire 374, resistor 364, wire 363, potentiometer 3&2, wire 361 to the negative point 354. This current introduced into the bridge circuit will have the effect of reducing the deflection of the galvanometer whenever the bridge goes to unbalance because the photosensitive cell 372 is conducting and is causing an energization of the heater load 22h. Thus, an anticipator action is had which prevents overshooting and hunting of th control, and enables a condition of balance to be more quickly reached in the bridge circuit.

In accordance with the present invention a set of values is provided for the components of the bridge 76 illustrated in BIG. 1, "but which the extent of error in degrees centigrade is the same for both the minimum and maximum values of temperature readings, or temperatures intended for the bulb 64 and which are to be measured. In the present instance these values are 50 degrees centigrade and 310 degrees centigrade.

The resistance values of the bridge components in order to obtain the said equal amounts of error in the low and high extremes of the readings are derived as 1 1 follows, reference being had to FIGS. 3 and 4 of the drawings. In these figures, the essential components of the bride 7d are shown, and the lead resistances corresponding to the wires 79 and 72 are indicated by the letter L, such resistances being represented by resistors '7 6a and 720 respectively in the circuit.

FIG. 3 indicates the setting of the slider 90 for the lowest temperature reading of 50 degrees, whereas FIG. 4 indicates the setting of the slider 90 for the maximum temperature reading of 310 degrees centigrade.

Applicant has found that it is possible for the maximum error, at either of these temperatures, to be less than 0.1 degree Centigrade for resistance values in each lead of from ohms to 2 ohms. For such changes in the lead resistance the extent of error in the low reading is the same as the extent of error at the high reading, and the error decreases for readings in between the said limits.

In FIGS. 3 and 4 the bridge components previously indentified by numbers have also been lettered for convenience in setting forth the equations by which the ohmic values are obtained.

Considering first FIG. 3, for a balanced condition of the bridge it will be seen that the following formulas hold:

Considering now the setting of the bridge indicated in FIG. 4, the following formulas hold:

The following equation, referring to the bridge network, sets forth the relations of the bridge components in order that the error (in degrees temperature) will be the same at both ends of the indicator setting, the letter B representing the temperature value at the high indicator setting and the letter b representing the tem- 0 perature value at the low indicator setting:

b(Ab) Let the quantity b(Ab) BMB) equalC Then, from (10) there is obtained:

From (4), substituting for Y, there is obtained:

From (8), substituting for Y, there is obtained:

(16) Z:A (KMX) +AX (17) Z=AK-AMX+AX Eliminating Z in Equations 15 and 17, there is had:

is aXK+MX=AK-AMX+AX Solving for X:

(19) X(a+AM-A+M):K+AK

Considering the bridge '76 set forth in FIG. 1 and utilizing the values already given, where the resistance of the bulb ranged from 107.39 ohms at degrees to 250.25 ohms at 310 degrees Centigrade the maximum error at these extreme temperatures was less than 0.1 degrees centigrade when changing the resistance of each lead from 0 to 2 ohms, as stated.

It will now be seen from the foregoing that Equations 4, 14, and 20 may be utilized to obtain the values of the resistors 80, 83 and 32 for a given resistor R and a given bulb having high and low resistance values of B and b respectively, and a given set of lead resistances.

Below are given values of the components of the cir cuits of P163. 1 and 2, and the above equations have been utilized in arriving at the resistance values of the bridge 76 illustrated in FIG. 1.

The heater 211 representing the load may have a power rating of 1500 watts. The voltage regulator 116 may be of the type known as Amperite No. 2H11. The secondary 113 of the transformer 116 provides 25 volts. The resistor 176 shunted across the lamps 143, 153' has a resistance of ohms. The series resistor 186 is 500 ohms, the series resistor 1158 is 450 ohms, the grid resistor 136 for the Thyratron has a resistance of 3500 ohms. The anticipator resistors 93 and 199 have 400 ohms resistance each.

The bridge leg 82 has a resistance of 2211 ohms. The adjustable bridge resistor 88 has a resistance of 132.82 ohms. The bridge resistor 84 has a resistance of 110.15 ohms. The Vernier adjustable resistor 86 has a resistance of 2 ohms. The bridge resistor 84 has a resistance of 2699 ohms. The rectifiers 128 and 130 may be of selenium or silicon, and may be of the kind known commercially as type M 20. The conductive photosensitive cell 1&8 may be of the type commercially available on the market under model No. CD5 10.

A re resentative set of values of resistance, voltage, current, power, etc. in connection with the circuit of FIG. 2 is as follows:

As with the circuit of FIG. 1, the load 2211 may be a 1500 watt electric heater. The transformer coil 238 has a 600 volt rating. The transformer coils 222, 224 are each rated at volts.

The filament transformer 260 is a Stancor model P4962. The transformer secondary 256 provides five volts. The vacuum tube 246 may be type 63213. The transformer 342 is a Dano 1425 V. DC. X23-3. The

13 grid resistor 364 has 20,000 ohms and a one watt rating. The rectifiers 334, 336 are type M 20.

The resistors 356 and each have 10,000 ohms resistance. The filter capacitor 352 is an electrolytic, 80 mfd. at 150 volts. The resistor 326 has 5,600 ohms and a rating of one Watt. The resistor 316 has 500 ohms, the resistor 330 has 20 ohms, the resistor 32% has 12 ohms, and the resistor 324 has 500 ohms resistance.

The rheostat 304- has 100 ohms resistance, and the resistor 324 has 500 ohms resistance. The rheostat 304 is adjusted to provide a current of 5.4 mils in the meter 293.

In the bridge, the resistors 290 and 292 are each 2680 ohms, carrying 2.7 milliamperes. The bridge resistor 400 is a six ohm slide wire potentiometer. The bridge resistor 394 has eight ohms.

In each of the circuits of FIGS. 1 and 2 the conductive condition of the photosensitive cell connects the control grid of the Thyratron tube directly to the positive terminal of the filter capacitor means whereby the negative charge on the grid is quickly drawn oil by and into the capacitor means with the full capacity of the condenser behind such action, which is independent of frequency, to quickly render the tube conducting so that a fast response characteristic is bad. Condenser discharge actions from one terminal of a capacitor, by which a pulse is delivered without attenuation from the presence of any series resistors connected to the terminal and wherein a single distinct return path is not readily observable, are known in connection with high voltages which effect a spark discharge. As at present understood the capacitor-backed grid discharge action of the signal circuit of FIGS. 1 and 2, unattenuated by any series resistor connected to the positive capacitor terminal, is somewhat of this nature except that lower voltages are present, of a value not capable of a jump discharge.

Variations and modifications may be made within the scope of the claims, and portions of the improvements may be used without others.

I claim:

1. In an electric temperature control, in combination, an electric heater and an AC. supply circuit and control means included therein for energizing the heater; a heatresponsive sensing element; a conductive-type photosensitive cell; means including a light source and an electrically energized light control device cooperable with said light source and connected with the sensing element, for selectively increasing and decreasing the illumination of the cell in response to cooling and heating of said element respectively; an electron valve device and energizing circuit therefor connected to the heater, said valve device having an anode connected to said control means, said anode receiving an A.C. polarization therefrom, said valve device further having a reference element, and having a control element whose positive and negative polarization with respect to the reference element respectively renders the valve device conducting and non-conducting to enerize and de-energize the heater; an energized control circuit connected to said A.C. supply circuit and including rectifiers operative with both negative and positive portions of the AC. cycle and filter capacitor means, said control circuit having a reference voltage point connected directly to said reference element, and having additional negative and positive voltage points providing full-wave negative and positive D.C. polarities with respect to said eference element, the positive voltage point comprising the positive terminal of the filter capacitor means; and a voltage drop device connected in series with the conductive cell, the junction of the drop device and cell being connected to the said control element and the remote points of the drop device and cell being connected respectively to the points of negative and positive polarity of the control circuit whereby the positive terminal of the filter capacitor means is joined to the cell and whereby the conductive and non-conductive conditions of the photosensitive cell eifect an instantaneous conduction and nonconduction of the electron valve device and consequently an instantaneous energization and de-energization of the heater respectively.

2. A control as in claim 1, in which the control circuit includes a center-tapped transformer winding, a pair of rectifiers and a filter capacitor connected as a full wave rectifier.

3. A control as in claim 1, in which the control circuit includes a transformer secondary and rectifiers connected to one end thereof and facing oppositely with respect to each other, and a pair of series-connected filter capacitors connected to the remaining ends of the rectifiers, whereby the latter and the capacitors comprise a closed series circuit, the remaining end of the transformer secondary being connected to the juncture of the capacitors and to the reference element of the valve device.

4, A control as in claim 1, in which the valve device has a heater-type cathode comprising the reference element, and in which there are time delay means for effecting a delay of operating potential on the valve device until the cathode has become heated.

5. A control as in claim 4, in which the light source comprises an electric lamp, and in which the time delay means is connected with said lamp and effects a delay in the energization of the lamp.

6. A control as in claim 4, in which the time delay means is connected with the control means and effects a delay in the AC. polarization of the anode of the valve device.

References Cited in the file of this patent UNITED STATES PATENTS 2,761,052 Knudsen Aug. 28, 1956 2,838,644 Rolfson et a1. June 10, 1958 2,909,926 Lingard Oct. 27, 1959 2,938,385 Mack et al. May 31, 1960 2,997,630 Kruse Aug. 22, 1961 

1. IN AN ELECTRIC TEMPERATURE CONTROL, IN COMBINATION, AN ELECTRIC HEATER AND AN A.C. SUPPLY CIRCUIT AND CONTROL MEANS INCLUDED THEREIN FOR ENERGIZING THE HEATER; A HEATRESPONSIVE SENSING ELEMENT; A CONDUCTIVE-TYPE PHOTOSENSITIVE CELL; MEANS INCLUDING A LIGHT SOURCE AND AN ELECTRICALLY ENERGIZED LIGHT CONTROL DEVICE COOPERABLE WITH SAID LIGHT SOURCE AND CONNECTED WITH THE SENSING ELEMENT, FOR SELECTIVELY INCREASING AND DECREASING THE ILLUMINATION OF THE CELL IN RESPONSE TO COOLING AND HEATING OF SAID ELEMENT RESPECTIVELY; AN ELECTRON VALVE DEVICE AND ENERGIZING CIRCUIT THEREFOR CONNECTED TO THE HEATER, SAID VALVE DEVICE HAVING AN ANODE CONNECTED TO SAID CONTROL MEANS, SAID ANODE RECEIVING AN A.C. POLARIZATION THEREFROM, SAID VALVE DEVICE FURTHER HAVING A REFERENCE ELEMENT, AND HAVING A CONTROL ELEMENT WHOSE POSITIVE AND NEGATIVE POLARIZATION WITH RESPECT TO THE REFERENCE ELEMENT RESPECTIVELY RENDERS THE VALVE DEVICE CONDUCTING AND NON-CONDUCTING TO ENERGIZE AND DE-ENERGIZE THE HEATER; AN ENERGIZED CONTROL CIRCUIT CONNECTED TO SAID A.C. SUPPLY CIRCUIT AND INCLUDING RECTIFIERS OPERATIVE WITH BOTH NEGATIVE AND POSITIVE PORTIONS OF THE A.C. CYCLE AND FILTER CAPACITOR MEANS, SAID CONTROL CIRCUIT HAVING A REFERENCE VOLTAGE POINT CONNECTED DIRECTLY TO SAID REFERENCE ELEMENT, AND HAVING ADDITIONAL NEGATIVE AND POSITIVE VOLTAGE POINTS PROVIDING FULL-WAVE NEGATIVE AND POSITIVE D.C. POLARITIES WITH RESPECT TO SAID REFERENCE ELEMENT, THE POSITIVE VOLTAGE POINT COMPRISING THE POSITIVE TERMINAL OF THE FILTER CAPACITOR MEANS; AND A VOLTAGE DROP DEVICE CONNECTED IN SERIES WITH THE CONDUCTIVE CELL, THE JUNCTION OF THE DROP DEVICE AND CELL BEING CONNECTED TO THE SAID CONTROL ELEMENT AND THE REMOTE POINTS OF THE DROP DEVICE AND CELL BEING CONNECTED RESPECTIVELY TO THE POINTS OF NEGATIVE AND POSITIVE POLARITY OF THE CONTROL CIRCUIT WHEREBY THE POSITIVE TERMINAL OF THE FILTER CAPACITOR MEANS IS JOINED TO THE CELL AND WHEREBY THE CONDUCTIVE AND NON-CONDUCTIVE CONDITIONS OF THE PHOTOSENSITIVE CELL EFFECT AN INSTANTANEOUS CONDUCTION AND NONCONDUCTION OF THE ELECTRON VALVE DEVICE AND CONSEQUENTLY AN INSTANTANEOUS ENERGIZATION AND DE-ENERGIZATION OF THE HEATER RESPECTIVELY. 